Device for determining and/or monitoring a process variable

ABSTRACT

The invention relates to a device for determining and/or monitoring a process variable. The aim of the invention is to provide a cost-effective, user-friendly device for determining and/or monitoring a process variable. The inventive device comprises the following elements: a sensor, a measuring/regulating/control unit ( 3 ) which pre-defines at least one event to be determined or monitored, and at least one memory unit ( 4; 5 ) which stores data according to the at least one pre-defined event. The sensor, the measuring/regulating/control unit ( 3 ) and the memory unit ( 4; 5 ) form a compact unit or an independent field appliance.

[0001] This invention relates to an apparatus for determining and/ormonitoring a process variable.

[0002] Prior-art measuring devices for determining and/or monitoring anarbitrary process variable (e.g., level, pressure, temperature, flowrate) in industrial measurement technology are so designed that only therespective current measured value is stored and made available. Fordiagnostic, error-detection, and predictive-maintenance purposes,however, the provision of the current measured value is very seldomsufficient. For diagnosis and for the detection of incipient faults, itis necessary to record measurement and/or system information over aprolonged period of time, so that it can be retrieved and evaluated whenrequired.

[0003] So far it has only become known to connect a so-called datalogger to the measuring device proper in case of need. The data loggeris capable of recording the measurement data over a desired period oftime for future reference. Commonly used data loggers are offered andsold by Endress+Hauser Wetzer GmbH+Co. KG, for example. These dataloggers are used, for instance, when a malfunction of the device ispresumed. Using the measurement and/or system data recorded by the datalogger over time, a fault diagnosis can be carried out. However, beforethe fault is actually located, much time may pass.

[0004] Error detection is particularly difficult if a malfunction occursonly intermittently, for instance at irregular intervals. In that caseit may happen that in the current measuring period, in which the datalogger is recording measurement and/or system data, the malfunction isnot detectable. The measuring device then operates error-free during therecording of the measurement data; it may happen, however, that themalfunction recurs in the near future and, in the extreme case, is notdetectable by a second check via the data logger, either.

[0005] Thus, the known analysis with the help of a data logger which isconnected to the measuring system only from time to time involves therisk that despite a great number of checks, an intermittently occurringmalfunction of the measuring device cannot be detected and,consequently, not be corrected. The method employed in the prior art istherefore uneconomical and may even be dangerous in criticalapplications. An example of a critical application is an intermittentlyoccurring fault in an overfill safeguard mounted in a tank which holdschemicals that are injurious to health.

[0006] The object of the invention is to provide a low-cost,operator-friendly, and reliable apparatus for determining and/ormonitoring a process variable.

[0007] This object is attained by an apparatus comprising a sensor, ameasuring/control unit which specifies at least one event to bedetermined or monitored, and at least one memory unit which stores dataas a function of the at least one predetermined event. In particular,the sensor, the measuring/control unit, and the memory unit form acompact unit or a self-contained field device. Since the apparatusaccording to the invention records measurement and system datacontinuously, arbitrary events can be diagnosed. Such an event is, forexample, a temporary or creeping malfunction of the measuring device. Assoon as an intermittent malfunction occurs, it can be detected with highreliability on the basis of the recorded data, and corrected. However,as will be explained later, the event may also be the current ortime-varying representation of a measured value.

[0008] In a further development of the apparatus according to theinvention, the process variable is a fill level, pressure, flow rate,temperature, density, conductivity, or any other physical or chemicalquantity to be measured. The event to be determined or monitored is,generally speaking, a predetermined time interval or a predeterminedtime scheme. Furthermore, again generally speaking, the event to bedetermined or monitored may be the attainment of a definedmeasured-value condition or the attainment of a defined system or faultcondition.

[0009] In a preferred embodiment of the apparatus according to theinvention, both the current measurement and/or system information andthe corresponding measurement and/or system information which occurredin a defined previous time range is stored in the memory unit. Thelatter measurement and system information will hereinafter be referredto as “history data”.

[0010] To enable the measuring/control unit to exchange data with aremote control station or to communicate with such a station, in afurther development of the apparatus according to the invention, a bussystem is provided. For the data exchange, any of the knowncommunications standards can be used, such as the Profibus PA standardor the Fieldbus Foundation standard. In particular, both the currentdata and the history data are transmitted over the bus system to theremote control station.

[0011] In another preferred embodiment of the apparatus according to theinvention, the memory unit for the history data is designed as aremovable compact unit. This embodiment makes it possible to use one andthe same memory unit for a multitude of measuring devices. This reducesthe cost of the apparatus according to the invention. The memory unit inthe compact unit may be, for instance an EEPROM, a hard disk, or avolatile memory device.

[0012] In a further development of the apparatus according to theinvention, the measuring/control unit has an associated interface viawhich an input/output unit or the memory unit for the history data isselectively connectable to the measuring/control unit. This developmentof the invention is particularly advantageous in that a memory unit canbe used in conjunction with a great number of apparatuses fordetermining and/or monitoring a process variable. In a preferredembodiment, the measuring/control unit carries out a fault analysisand/or a cause analysis and/or a prevention analysis using the historydata provided by the memory unit, and makes the results of the analysesavailable to the operating personnel. The analysis data is displayed tothe operating personnel on an on-site screen, for example.

[0013] Preferably, the output unit is designed to provide apixel-oriented display of current measurement data, intermediateresults, history data, and/or analysis data. In the case of levelmeasurements using a transit-time technique, for example, the currentmeasurement data is conveyed to the operating personnel in the form of aso-called echo curve. The echo curve embodies the amplitudes of the echosignals as a function of transit time or distance travelled. Of course,the display of the current measurement data derived by a transit-timetechnique also includes the display of a quantity derived from the echocurve. An example is a digital envelope. Typical echo curves are showsin FIGS. 6 and 7.

[0014] An interesting application of the display of history data is thetemporal variation of the level of a medium in a vessel. By means of thehistory data, it is also possible, of course, to detect and tracemeasurement disturbances.

[0015] The invention will now be explained in more detail with referenceto the accompanying drawings, in which:

[0016]FIG. 1 is a schematic representation of a level-measuring deviceusing the transit-time technique;

[0017]FIG. 2 is a schematic representation of a first embodiment of theapparatus according to the invention;

[0018]FIG. 3 is a schematic representation of a second embodiment of theapparatus according to the invention;

[0019]FIG. 4 is a schematic representation of a third embodiment of theapparatus according to the invention;

[0020]FIG. 5 is a schematic representation of a fourth embodiment of theapparatus according to the invention;

[0021]FIG. 6 shows a typical echo curve as is obtained with guidedmeasurement signals; and

[0022]FIG. 7 shows a typical echo curve as is obtained with freelyradiated measurement signals.

[0023]FIG. 1 shows schematically a level-measuring device 16.Level-measuring device 16 is mounted in an opening 14 in the lid 15 ofthe container 12. To determine the level of the medium 11 in thecontainer 12, radio-frequency measurement signals are guided along thesurface-wave transmission line 17 into the medium 11. A measurementsignal is shown in FIG. 1 as a radio-frequency pulse in stylized form.The measurement signals are generated in the signal-generating unit 18and coupled through the coupling unit 19 to the surface-wavetransmission line 17. The echo signals reflected from the surface 13 ofthe medium 11 are fed through the coupling unit 19 to themeasuring/control unit 3. From the signal transit time and the height ofthe container 12, the measuring/control unit 3 calculates, among otherthings, the level of the medium 11 in the container 12.

[0024]FIG. 2 shows schematically a first embodiment of the apparatusaccording to the invention. From the data made available by the sensor2, for example by the level sensor 16 shown in FIG. 1, themeasuring/control unit 3 determines the current measured values, i.e.,the current level of the medium 11 in the container 12, for example. Therespective current measurement data is stored in the memory unit 4. Overa defined period of time, the respective current measurement data iswritten into a memory unit for history data 5, the so-called historymemory. On the basis of the history data it is possible, for example, tocarry out a fault diagnosis or a cause analysis for short- and long-timemalfunctions of the level-measuring device. Furthermore, the historydata may be used for predictive-maintenance purposes.

[0025] In the embodiment shown, the measuring/control unit 3 isconnected via a bus system 6 to a remote control station 7. Via the bussystem 6, the sensor 2 or the measuring/control unit 3 and the controlstation 7 communicate with one another. The input/output unit 8 acts asan interface to the operating personnel: Here, data can be read out, newparameters can be entered, etc.

[0026] The history data is retrieved from the history memory 5 via themeasuring/control unit 3. Therefore, in the embodiment shown in FIG. 2,no additional terminals need be provided on the history memory 5. Theterminal may be located at any point of the bus system 6. The embodimentshown in FIG. 2 is suitable for use in hazardous areas, since thecommunication as such is designed for such use. However, certaindisadvantages arise from the fact that in this embodiment, the bussystem 6 is additionally loaded by the communication with the historymemory 5.

[0027] If the communication is to be speeded up, i.e., if more data isto be transmitted over the bus system 6 per unit time, a secondembodiment of the apparatus 1 according to the invention is appropriate,which is shown in FIG. 3. In this embodiment, the history memory 5communicates with the control station 7 via a separate bus system 6.Thus, the bus system (not shown in FIG. 3) between the measuring/controlunit 3 and the control station is not loaded by a superimposedcommunication with the history memory 5. A disadvantage of thisembodiment is, however, that an additional connector socket must beprovided; furthermore, additional access to the sensor 2 is necessary.In addition, the apparatus can only be used in hazardous areas if thesecond bus system 6 is also designed for use in hazardous areas. Fastercommunication is thus made possible at increased expense.

[0028]FIG. 4 shows schematically a third embodiment of the apparatusaccording to the invention, which can be used if on-line evaluation ofthe data is not necessary. In this embodiment, the memory unit 5 forhistory data is designed as a removable compact unit 9. This makes itpossible to send recorded data to any location for purposes ofevaluation and/or subsequent analysis. Another advantage of thisembodiment is that the communication over the bus system 6 is not loadedby the transfer of the history data. It goes without saying that theembodiment of the history memory 5 as a removable compact unit 9 canalso be used in conjunction with the other embodiments of the apparatusaccording to the invention.

[0029] A fourth embodiment of the apparatus according to the inventionis shown in FIG. 5. Here, the memory unit 5 for history data or aninput/output unit 8, e.g., an on-site display, can be selectivelyconnected to the measuring/control unit 3 via the interface 10. Theadvantage of this embodiment is that no additional connector socket needbe provided for the history memory 5. A minor disadvantage of thisembodiment is, however, that simultaneous operation of input/output unit8 and history memory 5 is not possible.

[0030] As stated above, the apparatus according to the invention offersa great number of advantages, particularly with regard to errordetection, detection of incipient errors (predictive maintenance), etc.For the first time, however, it also becomes possible to representmeasurement data graphically. In particular, the representation of theecho curve should be mentioned, which is evaluated if transit-timetechniques are used to determine and/or monitor the level of a materialin a container. On an on-site display 8 or at a remote control station7, the operator can visually follow the changes of the level in acontainer 12 by observing the shift of the useful-echo signal along thex-axis. This possibility of visualization will increase the operator'sconfidence in a technology in which level is detected via the transittime of radio-frequency measurement signals or of ultrasonic signals.

[0031] Of course, the invention also consists in the fact that thecurrent measurement data is displayed or made available at theinput/output unit 8. The current measurement data is preferably the echocurve.

[0032]FIG. 6 shows the typical echo curve of a TDR sensor. The echocurve, as already explained above, represents the echo amplitudes of ameasurement signal as a function of the distance travelled by themeasurement signal along the surface-wave transmission line 17, or as afunction of the corresponding transit time.

[0033] The first peak in the immediate vicinity of the origin ofcoordinates represents the so-called fiducial launcher. This peak iscaused by a step change in impedance and a resulting partial reflectionof the measurement signal at the interface between the coupling unit 19and the surface-wave transmission line 17.

[0034] The peak that is farthest from the origin is the end-of-linepeak, i.e., the peak representing that portion of the measurement signalwhich is reflected at the free end of the surface-wave transmission line17. The distinct peak between the fiducial launcher and the end-of-linepeak represents the useful-echo signal. The useful-echo signal is ameasure of the level of the medium 11 in the container 12. As a resultof the step change in impedance between two media—normally, these areair and a solid or liquid material stored in the container 12—a portionof the measurement signal is reflected. From the transit time or thedistance which is determined from the separation between a definedstarting point and the peak of the useful-echo signal, the level can bedetermined.

[0035] While the fiducial launcher and the end-of-line peak representsystem-dependent echo signals which exhibit no dependence on therespective level, the location of the useful-echo signal varies with therespective level: At a low level, the useful-echo signal will move inthe direction of the end-of-line peak; as the level rises, theuseful-echo signal will move in the direction of the fiducial launcher.

[0036] It goes without saying that in the case of freely radiatedmeasurement signals, the end-of-line peak in the echo curve does notoccur. However, a peak may occur which is caused by the reflection ofthe measurement signal from the bottom of the container. In that case,too, the location of the useful-echo signal varies with the level of themedium 11 in the container 12, of course. The peak of the useful-echosignal will therefore move along the x-axis, which represents either thetime or the distance travelled. The measurement signals freely radiatedvia an antenna may be ultrasonic or microwave signals. It is possible,of course, to use level-measuring devices 16 which are based on thepulse transit time technique or the FM-CW technique.

[0037] If the current echo curve is displayed on a pixel-oriented outputunit 8, a change in level is visually indicated to the operatingpersonnel. In many cases, this will increase the operating personnel'sconfidence in the measuring device, since a level change is indicatednot simply in the form of a change in a numerical value, but by means ofa signal which shifts spatially as a function of a varying level.

[0038] List of Reference Characters

[0039]1 Apparatus according to the invention

[0040]2 Sensor

[0041]3 Measuring/control unit

[0042]4 Memory unit for current data

[0043]5 Memory unit for history data

[0044]6 Bus system

[0045]7 Control station

[0046]8 Input/output unit

[0047]9 Removable compact unit

[0048]10 Interface

[0049]11 Medium

[0050]12 Container

[0051]13 Surface

[0052]14 Opening

[0053]15 Lid

[0054]16 Level-measuring device, level sensor

[0055]17 Surface-wave transmission line

[0056]18 Signal-generating unit

[0057]19 Coupling unit

1. An apparatus for determining and/or monitoring a process variable,comprising a sensor, a measuring/control unit (3) which specifies atleast one event to be determined or monitored, and at least one memoryunit (4; 5) which stores data as a function of the at least onespecified event, with the sensor, the measuring/control unit (3), andthe memory unit (4; 5) forming a compact unit or a self-contained fielddevice.
 2. An apparatus as claimed in claim 1 wherein the processvariable is a fill level, pressure, flow rate, temperature, density,conductivity, or any other physical or chemical quantity to be measured.3. An apparatus as claimed in claim 1 or 2 wherein the event to bedetermined or monitored is a predetermined time interval or apredetermined time scheme.
 4. An apparatus as claimed in claim 1 or 2wherein the event to be determined or monitored is the attainment of adefined measured-value condition or the attainment of a defined systemor fault condition.
 5. An apparatus as claimed in claim 1, 2, 3, or 4Wherein the memory unit (4, 5) store both the current measurement and/orsystem information and the corresponding measurement and/or systeminformation that occurred within a defined previous time range (→historydata).
 6. An apparatus as claimed in any one or more of the precedingclaims wherein a bus system (6) is provided via which themeasuring/control unit (3) or the memory unit (4; 5) communicates with aremote control station (7).
 7. An apparatus as claimed in claim 6wherein both the current data and the history data are transmitted overthe bus system (6) to the remote control station (7).
 8. An apparatus asclaimed in claims 1 to 5 wherein the memory unit (5) for the historydata is designed as a removable compact unit (9).
 9. An apparatus asclaimed in claims 1 to 5 wherein the measuring/control unit (3) has anassociated interface (10) via which an input/output unit (8) or thememory unit (5) for the history data is selectively connectable to themeasuring/control unit (3).
 10. An apparatus as claimed in any one ormore of the preceding claims wherein the measuring/control unit (3)carries out a fault analysis and/or a cause analysis and/or a preventionanalysis using the history data made available by the memory unit (5),and makes the results of the analyses available to the operatingpersonnel.
 11. An apparatus as claimed in claim 1 wherein the outputunit (8) is designed to provide a pixel-oriented display of currentmeasurement data, intermediate results, history data, and/or analysisdata.
 12. An apparatus as claimed in claim 11 wherein the display of thecurrent measurement data is the display of the echo curve made availableby a measuring device which determines the level of a medium in acontainer using a transit-time technique, or wherein the display of thecurrent measurement data is the display a quantity derived from the echocurve.
 13. An apparatus as claimed in claim 11 or 12 wherein the displayof history data is the display of the temporal variation of the level ofa medium (11) in a container (12).